1. Field of the Invention
The present invention relates to a dispersion compensating module which compensates for a dispersion of an optical transmission line in a signal wavelength band, and a line switching device including the dispersion compensating module and having a structure for performing line switching so that part of signals which propagate in an optical transmission line constituting part of an optical communication network in another optical transmission line, as well as an optical communication system which includes the dispersion compensating module.
2. Related Background Art
In an optical communication system, a single mode optical fiber is generally applied to an optical transmission line through which signals on a plurality of channnels propagate. This single mode optical fiber has a chromatic dispersion (hereinafter referred to simply as “dispersion”) of approximately zero near a 1.33 μm wavelength and a positive dispersion in a wavelength band of 1.3 μm, and a single mode is ensured in both the 1.3 μm wavelength band and 1.55 μm wavelength band. The single mode optical fiber is a silica-based optical fiber which includes a core which is doped with GeO2 or is made of pure silica, and a cladding doped with the F element. It is known that a transmission loss of this single mode optical fiber becomes the smallest in a 1.55 μm wavelength band.
In an optical communication system in which the single mode optical fiber is applied as an optical transmission line, a signal wavelength band makes use of a 1.3 μm wavelength band in which the dispersion of the single mode optical fiber can be reduced, or a 1.55 μm wavelength band in which the transmission loss of the single mode optical fiber is the smallest. Incidentally, since a fiber optic amplifier capable of amplifying signals of wavelength band 1.55 μm has been developed, the 1.55 μm wavelength band is often used as the signal wavelength band.
If the optical transmission line has a dispersion in the signal wavelength band, the waveform of each signal becomes degraded in accordance with to the propagation distance of the signals, and when the waveform degradation becomes large to some extent, there is a possibility that reception error occurs. To cope with this problem, a dispersion compensating module compensating for the dispersion is provided on the optical transmission line. The dispersion compensating module has a dispersion of sign different from that of the dispersion of the optical transmission line, and the dispersion of the optical transmission line is cancelled by the dispersion of this dispersion compensating modules whereby the degraded signal waveform is restored.
In the dispersion compensating module, if the dispersion compensation amount can be set on the basis of the degree of waveform degradation of each signal which propagates through the optical transmission line, the degraded signal waveform can be restored irrespective of the degree of degradation of the signal waveform. For example, when signals suffer a large waveform degradation while the signals propagate through a long-haul optical transmission line, the dispersion compensating module reforms the greatly degraded signal waveforms by setting the dispersion compensation amount to an increased amount. On the other hand, when signals suffer a small waveform degradation while the signals propagate through a short-haul optical transmission line, the dispersion compensating module restores the slightly degraded signal waveforms by setting the dispersion compensation amount to a decreased amount. Several kinds of dispersion compensating modules each of which is variable in its dispersion compensation amount have been proposed.
For example, Japanese Patent Laid-Open No. 251973/1999 proposes a dispersion compensating module which compensates for the dispersion of test light outputted separately from signals transmitted from a transmitting section by means of a variable dispersion compensator, detects the degree of waveform distortion of the dispersion-compensated test light, and provides feedback control over the dispersion compensation amount of the variable dispersion compensator on the basis of the detected degree of waveform degradation. Owing to this construction, the dispersion compensating module can restore the degraded signal waveform irrespective of the degree of waveform degradation of each signal outputted from the transmitting section.
Japanese Patent Laid-Open Nos. 88261/1999 and 68657/1999 propose a dispersion compensating module which detects the strength of dispersion-compensated signals of a specific wavelength which exists in a signal wavelength band, and provides feedback control over the dispersion compensation amount of a variable dispersion compensator to maximize or minimize the detected signal intensity of the specific wavelength.
Japanese Patent Laid-Open No. 41891/1998 proposes a dispersion compensating module which has a structure in which an annular optical fiber is optically coupled via an optical switch to an optical fiber connected to an optical transmission line, and which adjusts the dispersion compensation amount by the on-off switching of the optical switch.
In addition, Japanese Patent Laid-Open No. 88260/1999 proposes a dispersion compensating module which includes a fixed dispersion compensating section having a fixed dispersion compensation amount and a variable dispersion compensating section having a variable dispersion compensation amount so that the dispersion of an optical transmission line is roughly compensated for by the fixed dispersion compensating section and is precisely compensated for by the variable dispersion compensating section.